The present invention is an improvement of the switch disclosed in U.S. Pat. No. 4,725,741. The Darlington connected power switch disclosed in the '741 patent achieves rapid turn-off. A pair of turn-off switches respectively control the turn-off of the Darlington connected power transistors by applying a negative base drive having a predetermined RC time constant to each power transistor. The RC time constants are chosen as a function of the switching characteristics of the Darlington connected power transistor switches. Characteristics of power transistor switches vary from manufacturer to manufacturer which requires the aforementioned time constants to be tuned to the particular power transistors being utilized. The aforementioned tuning makes it difficult to use a standard design which is disadvantageous.
Tuning of the time constants is necessary to achieve proper operation of the Darlington connected power transistor switches. Under light load conditions without appropriately chosen time constants, the driver transistor is subject to turn-off at a slower rate than the driven transistor. As a result of the slower turn-off of the driver transistor, the driven transistor is subject to turning back on as a consequence of the base drive of the driven transistor not being totally extinguished prior to turn-off of the driven transistor. The aforementioned tuning of a time constant in the turn-off switches of a Darlington connected switch of the '741 patent eliminates the aforementioned problem of turning back on the driven transistor as a consequence of slow turn-off of the driver transistor at the expense of requiring a different RC time constant to be chosen for transistors which are chosen for driving loads of different power requirements.
U.S. patent application Ser. No. 135,226, filed on Dec. 21, 1987, and assigned to the assignee of the present invention, discloses a Darlington connected switching circuit having a MOSFET as the driver transistor and a bipolar transistor as the driven transistor. Turn-off of the Darlington switch is not controlled as a function of the collector to emitter potential of the driven transistor as in the present invention.
U.S. Pat. No. 4,410,810 discloses a high speed switching circuit having a single bipolar transistor which monitors the collector to emitter potential of the transistor to sense when the transistor collector to emitter potential has reached a predetermined value indicative of the transistor being out of saturation. Negative base drive is applied to the bipolar transistor in response to sensing that the transistor collector to emitter potential has reached a predetermined value.